This invention relates to apparatus mountable to pressurized liquid hydrocarbon storage tanks for remote monitoring of the volume of contents therein, and more particularly to improvements in monitors which function by suspending a buoyant displacement probe in liquid tank contents from a load cell to measure the apparent weight of the probe.
Storage tanks for liquefied hydrocarbon products such as butane and propane present special problems for the safe and ready monitoring of tank volume levels. This is particularly so where the tanks are located in remote or relatively inaccessible locations, making tank inspections inefficient and inconvenient.
LP gas storage tanks are classified as explosion hazards by the National Fire Protection Association, requiring special care in the design and installation of any ancillary equipment. The LP Gas Code(NFPA 58) defines the area within 5 feet of any tank, fill opening or point where LP gas is dispensed, loaded, vented or the like as a Class I, Division 1, Group D hazard.
Despite the daunting nature of the problems involved in safely installing a volume monitor for such a tank, it would be desirable to provide for the remote reading of volume levels so that fuel supplies can be maintained in adequate amounts without the necessity of site trips to ascertain the amount of fuel on hand.
It is known in the measuring art to determine the weight of liquid contents in a tank by suspending a buoyant probe in the liquid from a load cell to measure the apparent weight of the probe. By well-known calculational formulas, such measurement yields the weight of tank contents. Examples of such systems in the prior patent art may be found in U.S. Pat. Nos. 5,614,672, 5,157,968, 5,132,923 and 4,244,218.
U.S. Pat. No. 5,157,968 discloses a buoyant displacement probe mounted through a top tank port via a load cell for the determination of the liquid tank content weight. It also provides a second buoyant probe mounted via a load cell through a second tank port, so that the specific gravity of the liquid may be calculated from the second load cell reading and the content weight thus be converted to a volume. U.S. Pat. No. 5,614,672 likewise determines content weight by a load cell-mounted buoyant displacement probe. It, however, relies for volume determination on an assumption that the specific gravity of the liquid in the tank is a constant, known value.
Despite these efforts to provide tank monitors employing buoyant displacement probes for monitoring tank contents, there remains a lack of suitable devices employing this principle for safe and ready installation on remote hazardous storage tanks. This invention is directed to apparatus which will satisfy this need by providing an accurate and reliable monitoring system which can be safely installed for monitoring of the liquid contents of hazardous storage tanks.
The object of this invention is to provide a volume monitoring apparatus which may be readily and safely installed on both new and existing tanks used to store liquid propane, liquid butane or like hazardous liquids. An important consideration in this invention is to maintain a high degree of accuracy in the volume determination.
In accordance with the invention, there is provided a device for measuring the liquid volume in a tank, including a load cell positioned adjacent a port in the top wall of the tank. A downwardly extending displacement probe is supported by the load cell, and has sufficient length so that its lower end is adjacent to, but not touching, the tank bottom. A suspension mechanism for the load cell includes a universal joint means allowing the load cell to be oriented horizontally without regard to the orientation of the tank and port. The load cell monitors the apparent weight of the probe, and thus the weight of the liquid contents in the tank. An elongate temperature probe is mounted in the same port and has at least one temperature sensor for measuring the temperature of the liquid in the tank. This temperature reading may then be used with the known specific gravity-temperature curve of the stored liquid to utilize the specific gravity of the stored liquid at the measured temperature to calculate the volume of the liquid content.
In a specific embodiment of the invention, the displacement probe is an elongate hollow member. The temperature probe is a string extending through the open center of the displacement probe, the string carrying a plurality of spaced thermometric units for measuring the temperature of the tank liquid at spaced levels of the tank. The preferred probe of the invention is in modular form, comprising plural discrete segments which may be assembled on site during installation of the monitor by the use of joining collars, one positioned between each adjacent segment.
The preferred form of the invention includes a riser pipe secured in the tank port, a mounting flange threaded on the riser pipe and a flange cap secured to the flange to close the port, the flange cap supporting a hanger bracket from which the load cell and displacement probe are suspended by way of a universal joint assembly which insures that the load cell, in the form of a planar beam sensor is maintained in true horizontal orientation.